Olefin Metathesis inWater: Speciation of a LeadingWater-Soluble Catalyst Pinpoints Challenges and Opportunities forChemical Biology

The metathetical modification of biomolecules in aqueous environments holds great promise for advances at the interface of chemistry, biology, and medicine. However, rapid degradation of the metathesis catalysts necessitates their use in large stoichiometric excess, resulting in undesired side-reactions promoted by the ruthenium products. Although water is now known to play a central role in catalyst decomposition, the elusive nature of the intermediates has hampered insight into the pathways involved. We describe the detailed speciation in water of AquaMet (<b>AM</b>), the dominant ruthenium catalyst used for aqueous metathesis, and implications for catalysis. Potentiometric and spectroscopic speciation studies reveal that only trace <b>AM</b> is present under the pH-neutral, salt-free conditions routinely employed in synthetic applications of aqueous metathesis. Instead, metathesis-inactive hydroxide species dominate. Even at pH 3, Ru–H₂O complexes dominate in 0.01 M NaCl_(aq), and the water ligands are readily deprotonated as the pH is increased. Raising NaCl_(aq) concentrations to 1 M suppresses deprotonation events below pH 8, stabilizing <b>AM</b> as the dominant solution species at neutral pH, and significantly expanding the metathesis-compatible regime. Hitherto unrecognized catalyst solubility issues are also revealed, pointing toward avenues for advance. More broadly, the capacity to directly link catalyst environment to structure and performance opens new opportunities for olefin metathesis in complex, water-rich settings.